Kinetic Trapping of D2 in MIL-53(Al) Observed Using Neutron Scattering

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• 作者：Rachel A. Pollock ; Jae-Hyuk Her ; Craig M. Brown ; Yun Liu ; Anne Dailly
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：August 7, 2014
• 年：2014
• 卷：118
• 期：31
• 页码：18197-18206
• 全文大小：542K
• ISSN：1932-7455

文摘

We have studied gas adsorption effects on the structure of a metal鈥搊rganic framework, MIL-53(Al), a material well-known because of the controllable framework 鈥渂reathing鈥?phenomenon. Neutron powder diffraction between 4 and 77 K and up to 4.5 bar pressure D₂ confirms that a structural phase transition is responsible for the observed H₂/D₂ isotherm hysteresis at 77 K. We find two crystallographically distinct D₂ adsorption sites in MIL-53(Al) when the pores are fully opened, similar to those reported for D₂ in MIL-53(Cr), but in contrast with the previously published cases of CO₂ and H₂O. Upon desorption of D₂ at 77 K, we find strong evidence for the existence of D₂ molecules kinetically trapped in the center of the closed pore of MIL-53(Al). This 鈥渕olecular clamp鈥?appears to be functional until 鈮?20 K, the temperature at which the D₂ eventually desorbs under dynamic vacuum. Hydrogen diffusion constants calculated using quasielastic neutron scattering data collected along the isotherm are also consistent with H₂ being trapped in the closed pore structure. Diffraction experiments performed with N₂ and He gases under similar conditions show the range of structural response from immediate pore opening at low N₂ pressures (<1 bar) to no observable effect at 10 bar He.